This paper describes POCO's new capability to rapidly produce large silicon carbide mirror substrates by conversion joining segments of silicon carbide during the process of converting graphite to silicon carbide. Mirror segments and structures are machined from a special graphite and subsequently joined together during the conversion process with the end result being a high purity beta silicon carbide structure. Interface boundaries are removed by the crystal growth across boundaries as the graphite crystal structure is converted to the larger crystal size of silicon carbide. Results of conversion joining development, design guidelines and limitations of the conversion joining process will be presented.

SSG Precision Optronics, Inc. (SSG) has recently developed a number of Reaction Bonded (RB) Silicon Carbide (SiC) optical systems for space-based remote sensing and astronomical observing applications. RB SiC's superior material properties make it uniquely well suited to meet the image quality and long term dimensional stability requirements associated with these applications. An overview of the RB SiC manufacturing process is presented, along with a summary description of recently delivered RB SiC flight hardware. This hardware includes an RB SiC telescope and Pointing Mirror Assembly (PMA) for the Geostationary Imaging Fourier Transform Spectrometer (GIFTS) mission and an imaging telescope for the Long-Range Reconnaissance Imager (LORRI) mission. SSG continues to advance the state-of-the-technology with SiC materials and systems. A summary of development activities related to a low-cost, fracture tough, fiber reinforced RB SiC material formulation, novel tooling to produce monolithic, partially closed back mirror geometries, and extension of the technology to large aspheric mirrors is also provided.

SSG Precision Optronics, Inc. has delivered a silicon carbide (SiC) pointing mirror and telescope for NASA's Geostationary Imaging Fourier Transform Spectrometer (GIFTS) project. The 28 x 45 cm SiC pointing mirror is part of SSG's two-axis gimbaled mirror assembly that will provide object-space pointing and jitter control. The 24 cm aperture telescope is an off-axis afocal three mirror anastigmat that is the collection aperture for the GIFTS instrument. Silicon carbide was selected for the GIFTS pointing mirror and telescope in order to minimize weight, provide athermal optical performance from room temperature to 190 Kelvin, and maintain image quality and line-of-sight stability in the presence of partial or full solar loading (minimizing solar outages). Both subsystems were successfully designed, fabricated, and subjected to testing prior to being delivered to Utah State University's Space Dynamics Laboratory for integration. This paper describes the pointing mirror and telescope design and hardware results.

The following is an overview on sintered silicon carbide (SSiC) material properties and processing requirements for the manufacturing of components for advanced technology optical systems. The overview will compare SSiC material properties to typical materials used for optics and optical structures. In addition, it will review manufacturing processes required to produce optical components in detail by process step. The process overview will illustrate current manufacturing process and concepts to expand the process size capability. The overview will include information on the substantial capital equipment employed in the manufacturing of SSIC.
This paper will also review common in-process inspection methodology and design rules. The design rules are used to improve production yield, minimize cost, and maximize the inherent benefits of SSiC for optical systems. Optimizing optical system designs for a SSiC manufacturing process will allow systems designers to utilize SSiC as a low risk, cost competitive, and fast cycle time technology for next generation optical systems.

Direct Sintered Silicon Carbide (SSiC) is a promising material for mirror optics due to its low density, high stiffness and high thermal stability. In order to make large mirror optics (over 1 meter diameter), processing limitations to create monolithic structures of this size class require that smaller segments need to be fabricated and then joined in a post sintering operation. Fabrication of segmented &nullset;300mm lightweighted concave mirrors to demonstrate different fabrication methods is presented here. The mirrors are comprised of 6 radial segments joined by means of silicon braze technology and are coated with a SSiC Chemical Vapor Deposition (CVD) layer for improved surface finish to reduce straylight scatter. Evaluation of conventional pitch lap polishing of brazed and coated optic surfaces has shown no degradation to surface figure and surface roughness.

The chemical vapor composites (CVC) process provides for the rapid manufacture of near net shape, reduced residual stress silicon carbide (SiC) suitable for high performance optics. The reduction or elimination of residual stress provides several key advantages: 1) increased growth rate, 2) high yields, and 3) near net shape deposition of complex geometries. Near net shape deposition allows for fabrication of spherical and aspherical optics without machining of the optical surface. Final surface figures of optical flats are typically better than 1/10λ (P-V) and 2-5&angst; surface roughness. A comparison of ultraviolet spectrum reflectance of CVC SiC and that of single crystal SiC is discussed. The complex optical constants of CVC SiC in the mid-infrared spectrum are also presented.

Cesic is a ceramic matrix composite material, developed by ECM, that is characterized by high stiffness, high thermal conductivity, and low thermal expansion. These characteristics make it an ideal material for large high-precision optical and structural applications. GE Energy has acquired the rights to manufacture Cesic in the USA as ECM continues to manufacture it. GE Energy has demonstrated the ability to produce equivalent material properties with densities of 2.65 gm/cm3 and four-point bending strength of 110 MPa on large samples (150 mm length). The status of technology transfer and examples of mirrors and structures manufactured by GE Energy and ECM will be presented.

SiC optics has been considered for a very long time. Today, there are a few military and commercial applications. Future imaging and energy transfer applications require robustness on a par with metallic systems. Intrinsic, low fracture toughness of several classes of monolithic SiC is the key impediment in these applications. A new form of SiC-SiC composite for optical applications has been developed. It features high modulus combined with high fracture toughness. This new, highly innovative technology offers the potential in demanding government applications, as well as large surveillance optics (increased toughness can translate into lower aerial density) and high energy commercial lasers. SiC-SIC is a novel technology for optical structures consisting of integrated composite materials and structures which exhibits excellent fracture toughness and homogeneous CTE.

Cesic -- a trademark of ECM for carbon-fiber reinforced silicon carbide -- allows relatively quick and cheap manufacturing of components with good repeatability and reproducibility due to the implemented product assurance (PA) system. Through a joining process and our development of optical surfaces, Cesic allows for a direct up-scaling of structures and optical surfaces to large-size applications and systems. The size of the structures and mirrors that can be manufactured is limited only by the scale of the available furnaces, the largest of which currently is 2.4 m in diameter. Under ESA contract ECM performed a measurement program to get reliable material properties data on Cesic for space and other future programs.

Under ESO - European Southern Observatory - contract ECM has performed a feasibility study for the manufacturing of Cesic primary and secondary mirror segments for the OWL-Telescope. The main issues of this study were to demonstrate the feasibility of the serial production (~ 2550 segments) of Cesic mirror segments under a certain schedule and cost optimisation aspect for the segments. Part of this study was also a pre-design of a manufacturing facility for this big amount of mirror segments. This study is limited only up to the manufacturing of a polishable surface, the feasibility of the polishing capability is not part of this study.

New-Technology Silicon Carbide (NT-SiC) is a reaction sintered silicon carbide with very high bending strength. Two times higher bending strength than other SiC materials is important characteristics in an optical mirror for space application. The space optics is to endure the launch environment such as mechanical vibration and shock as well as lightweight and good thermal stability of their figure. NT-SiC has no open pore. It provides good surface roughness for infrared and visible application, when its surface is polished without additional coatings. Additional advantages are in the fabrication process. The sintering temperature is significantly lower than that of pure silicon carbide ceramics and its sintering shrinkage is smaller than one percent. These advantages will provide rapid progress to fabricate large structures and will enable that one meter mirror will put practical use. It is concluded that NT-SiC has potential to provide large lightweight optical mirror.

Silicon carbide (SiC) is the most advantageous as the material of various telescope mirrors, because of high stiffness, low density, low coefficient of thermal expansion, high thermal conductivity and thermal stability. Newly developed high-strength reaction-sintered silicon carbide (NTSIC), which has two times higher strength than sintered SiC, is one of the most promising candidates for lightweight optical mirror substrate, because of fully dense, lightweight, small sintering shrinkage (±1 %), good shape capability and low processing temperature. In this study, 650mm in diameter mirror substrate of NTSIC was developed for space telescope applications. Three developed points describe below. The first point was to realize the lightweight to thin the thickness of green bodies. Ribs down to 3mm thickness can be obtained by strengthen the green body. The second point was to enlarge the mirror size. 650mm in diameter of mirror substrate can be fabricated with enlarging the diameter in order. The final point was to realize the homogeneity of mirror substrate. Some properties, such as density, bending strength, coefficient of thermal expansion, Young's modulus, Poisson's ratio, fracture toughness, were measured by the test pieces cutting from the fabricated mirror substrates.

For some years Alcatel Space has been interested in the development of a new material to produce lightweight, stiff, stable and cost effective structures and mirrors for space instrument. Cesic from ECM has been selected for its intrinsic properties (high specific modulus, high conductivity, quite low thermal expansion coefficient and high fracture toughness for a ceramic material), added to ample manufacturing capabilities. Under ESA responsibility, a flight representative optical bench of Cesic has been designed, manufactured and tested. The optical bench has been submitted with success to intensive vibration tests up to 80 g on shaker without problem and was tested down to 30 K showing very high stability. Cesic is also envisaged for large and lightweight space telescope mirrors. Coatings on the Cesic substrate have been developed and qualified for the most stringent optical needs. To prove the lightweight capability, a large Cesic mirror D=950 mm with an area mass of less than 25 kg/m2 has been designed, sized again launch loads and WFE performance, and then manufactured. Cesic is also envisaged for large future focal plane holding a large number of detectors assuring high stability thanks to its high thermal conductivity. A full size Cesic focal plane has been already successfully built and tested. Based on these successful results, Alcatel Space is now in position to propose for space projects this technology mastered in common with ECM both for mirrors and structures with new innovative concepts thanks to the manufacturing capabilities of this technology.

BOOSTEC has pioneered, since its creation in 1999, the market of space optics made of ceramics (silicon carbide). BOOSTEC has recently delivered the Herschel space telescope, which is, with its 3.5 m diameter mirror, the largest space telescope ever made in the world. The following paper will review the current status of high performing silicon carbide components for space optics. It will then focus on the new possibilities under development, like focal planes and complex systems. It will finally highlight new areas of development for material, process, and applications.

One of the key technologies for next generation space telescope with a large-scale reflector is a material having high specific strength, high specific stiffness, low coefficient of thermal expansion and high coefficient of thermal conductivity. Several candidates such as fused silica, beryllium, silicon carbide and carbon fiber reinforced composites have been evaluated. Pitch-based carbon fiber reinforced SiC composites were developed for the SPICA space telescope mirror to comply with such requirements. Mechanical performance such as bending stiffness, bending strength and fracture toughness was significantly improved. Evaluation procedures of thermal expansion and thermal conductivity behavior at cryogenic temperatures (as low as 4.5K) were established and excellent performance for the SPICA mirror was demonstrated.

The Terrestrial Planet Finder mission requires extreme dynamic stability at cryogenic temperatures in order to carry out its objectives of searching for and observing extraterrestrial planets. As a result, the ability to meet its ambitious science goals will be significantly enhanced by increasing its vibrational damping at cryogenic temperatures. Given the low inherent structural damping at cryogenic temperatures, significant reduction in vibration amplitude could be gained with only modest increases in damping on the structure. To examine the use of vibrational damping options to improve the dynamic stability of cryogenic structures, Jet Propulsion Laboratory has conducted a series of experiments to measure the damping levels of various materials at cryogenic temperatures and to search for the materials with higher cryogenic damping. This paper summarizes our experimental observations on the material damping of silicon foam and silicon carbide foam materials at cryogenic temperatures. These foam materials have been independently developed by Schafer Corporation and have properties that enable their applications in space environments with a range of temperature from 25K to 500K. These materials have been used for mirrors, and uses for foam based structures such as optical mounts and benches are currently in development. As observed from the measured damping, these two foam materials have higher damping than aluminum at cryogenic temperatures, and the damping level is relatively insensitive to temperature change from room to cryogenic temperatures. As a result, these materials may be potential candidates to achieve increased levels of cryogenic damping for the Terrestrial Planet Finder mission.

Silicon and silicon carbide foams provide the lightweighting element for Schafer Corporation's silicon and silicon carbide lightweight mirror systems (SLMSTM and SiC-SLMSTM). SLMSTM and SiC-SLMSTM provide the enabling technology for manufacturing lightweight, athermal optical sub-assemblies and instruments. Silicon and silicon carbide foam samples were manufactured and tested under a Schafer-funded Internal Research and Development program in various configurations to obtain mechanical and thermal property data. The results of the mechanical tests that are reported in this paper include Young's modulus, compression strength, tensile strength, Poisson's ratio and vibrational damping. The results of the thermal tests include thermal conductivity and coefficient of thermal expansion.

A major design and analysis challenge for the JWST ISIM structure is thermal survivability of metal/composite adhesively bonded joints at the cryogenic temperature of 30K (-405°F). Current bonded joint concepts include internal invar plug fittings, external saddle titanium/invar fittings and composite gusset/clip joints all bonded to hybrid composite tubes (75mm square) made with M55J/954-6 and T300/954-6 prepregs. Analytical experience and design work done on metal/composite bonded joints at temperatures below that of liquid nitrogen are limited and important analysis tools, material properties, and failure criteria for composites at cryogenic temperatures are sparse in the literature. Increasing this challenge is the difficulty in testing for these required tools and properties at cryogenic temperatures. To gain confidence in analyzing and designing the ISIM joints, a comprehensive joint development test program has been planned and is currently running. The test program is designed to produce required analytical tools and develop a composite failure criterion for bonded joint strengths at cryogenic temperatures. Finite element analysis is used to design simple test coupons that simulate anticipated stress states in the flight joints; subsequently, the test results are used to correlate the analysis technique for the final design of the bonded joints. In this work, we present an overview of the analysis and test methodology, current results, and working joint designs based on developed techniques and properties.

Three samples of Schott Zerodur were recently measured using Jet Propulsion Laboratory's Cryogenic Dilatometer Facility. The initial purpose of these tests was to provide precision CTE measurements to help correlate thermomechanical models with the actual performance of NASA's Space Interferometry Mission (SIM) TOM-1C testbed. A total of six Zerodur test samples, as well as the SIM testbed mirror were machined from the same block of glass. Thermal strain as a function of time, sample temperature, and cooling rate were measured over a temperature range of 270K to 310K. Presented in this paper is a discussion of the sample configuration, test facilities, test method, data analysis, test results, and future plans.

Linear thermal expansion measurements of nine samples of Lead Magnesium Niobate (PMN) electroceramic material were recently performed in support of NASA's Terrestrial Planet Finder Coronagraph (TPF-C) mission. The TPF-C mission is a visible light coronagraph designed to look at roughly 50 stars pre-selected as good candidates for possessing earth-like planets. Upon detection of an earth-like planet, TPF-C will analyze the visible-light signature of the planet's atmosphere for specific spectroscopic indicators that life may exist there. With this focus, the project's primary interest in PMN material is for use as a solid-state actuator for deformable mirrors or compensating optics. The nine test samples were machined from three distinct boules of PMN ceramic manufactured by Xinetics Inc. Thermal expansion measurements were performed in 2005 at NASA Jet Propulsion Laboratory (JPL) in their Cryogenic Dilatometer Facility. All measurements were performed in vacuum with sample temperature actively controlled over the range of 270K to 310K. Expansion and contraction of the test samples with temperature was measured using a JPL-developed interferometric system capable of sub-nanometer accuracy. Presented in this paper is a discussion of the sample configuration, test facilities, test method, data analysis, test results, and future plans.

Shape memory composite materials (SMC materials) are being developed by our program to make deployable space optics. The basic procedure involves electroforming an approximately 20 micron thin Ni surface onto a convex master and then casting the shape memory composite material onto the plated master. When good adhesion between the Ni and the SMC material is obtained, the Ni and SMC material come off the master in one piece. The result is a shiny mirror whose metallic surface remains intact after stowing and deploying of the mirror. Achieving the requisite adhesion requires treating the Ni prior to the application of the SMC material. The techniques we use to treat the Ni and the results of making mirrors are described.

The quickest method for generating a lightweight composite optic is to replicate an optical quality glass tool onto a carbon fiber reinforced polymer (CFRP). However, the effects of fiber print-through create an unacceptable surface roughness on replicated CFRP mirrors. In order to mitigate fiber print-through, two methods of generating a polishable resin layer were investigated. The first method employs the application of resin films to the CFRP surface. The second, unconventional method generates a co-cured resin layer using a magnetic fiber migration approach. A final polishing step was used to attain optical quality surface features on all of the replicated specimens. Replicated resin films with thicknesses ≥ 0.25 mm sufficiently mitigate fiber print-through. Room temperature and high temperature cure resins were polished below 50 &angst; rms surface roughness (1 μm to 1mm bandwidth) or better. The magnetic fiber migration technique was suitable for eliminating fiber print-through. Replicated magnetic fiber laminates were polished to within specular quality as well.

Glass, crystals, and brittle ceramics are subject to time and moisture related failures due to stress corrosion, or slow crack growth. Equations governing time to failure are well established, but presume knowledge of crack velocity, flaw shape factor, and initial stress intensity factors that are often unavailable. Alternative crack growth data, obtained from dynamic fatigue testing, is often used, but data of stressing rates and moist strength are necessary for calculation, and again are often unavailable. Programs that are available to calculate failure times presume such knowledge as well. A complication results in use of any available data in these programs, in that residual stress in the material of interest is not included. Tests show that residual stress is present even for polished surfaces, and such stresses greatly reduce failure times, often by several orders of magnitude. Further, such programs presume a completely moist environment, which is often not the case for spaced based or airborne systems. To overcome these difficulties, an approximate method of computing margin of safety in glass and other brittle ceramics under stress, time, and moisture is provided when only flaw growth fatigue resistance parameter and inert strength are known but details of crack velocity and initial crack depth are unknown. The method includes effects of relative humidity and residual stress, and is readily programmed to a spreadsheet to aid in quick computation. Using appropriate safety factors as herein defined, the method is compared to the exact formulation for several cases in which velocity data are known. Results compare with quite favorable accuracy, precluding the need for costly fracture mechanics data and unwieldy computation.

The low thermal expansion glass ceramic ZERODUR is the material of choice for many big astronomical telescope projects like VLT, Keck I + II, HET, LAMOST and GRANTECAN (GTC). For future giant telescope projects like OWL or TMT with at least several hundreds of mirror blanks the CTE homogeneity within a single blank and from blank to blank is an crucial issue.
The ZERODUR production process is based on established and proven methods used in the production of high homogeneity optical glasses. Therefore ZERODUR itself is a material of highest homogeneity even in large dimensions and huge quantities. This paper presents an evaluation of the homogeneity of the thermal expansion coefficient within more than 250 mirror blanks. The observed homogeneity range is only slightly larger than the repeatability of the standard dilatometer measurement of ±0.005*10-6 K-1.
To improve the accuracy of measurement and to get a deeper understanding of the thermal expansion behaviour of ZERODUR a new dilatometer was built exhibiting a repeatability of ±0.001*10-6 K-1. Detailed evaluations of the thermal expansion coefficient homogeneity of a 100 mm x 100 mm ZERODUR test block showed no variation within the repeatability of measurement of the improved dilatometer.

Material properties and performances for near future applications were studied for low thermal expansion glass-ceramics; CLEARCERAM series produced by OHARA Inc. In the material study, the improved CTE characteristics of CLEARCERAM-Z HS over conventional CLEARCERAM-Z were shown with interferometric CTE metrology system with 2ppb/degree C repeatability. Also the material uniformity was evaluated by means of refractive index homogeneity. In the application study, the capabilities of CLEARCERAM-Z HS for near future precision applications, Extreme Ultra Violet Lithography (EUVL) system component and extremely large telescope mirror applications were discussed by reviewing actual data prepared in accordance with the existing specifications for each application. For EUVL system component application, inter lot CTE uniformity and surface finish data meeting the SEMI P37 specification were presented and the performance was evaluated. For extremely large telescope mirror application, intra disk uniformity of CTE and stress birefringence for Dia.780mm CLEARCERAM-Z HS meeting mirror blanks specification of existing extremely large telescope project were demonstrated and the potential performance of CLEARCERAM-Z HS for the application was shown with the on-going integration of the size availability at OHARA Inc.

Typically, optical molds have been made from silicon carbide (SiC) or tungsten carbide (WC). Magnetorheological Finishing (MRF) polishing results of SiC and WC molds will be reviewed. Impressive figure corrections have been demonstrated on both types of materials. The roughness performance of CVD-SiC, WC and binderless WC will be compared. However, the hardness and polycrystalline nature of these materials make them difficult to manufacture. In this paper we report positive initial results using an alternate mold material, glassy carbon. Test samples have been ground, pre-polished and finish polished to a 38 nm surface figure peak-to-valley (PV) and a 6 &angst; rms surface roughness, with improved cycle times versus SiC and WC. Glassy carbon is a promising mold material candidate as an amorphous material of lower hardness. The lower hardness leads to more effective diamond grinding process and results in a better surface rms roughness following MRF. After reviewing key material properties of glassy carbon material, this paper will describe some collaborative activities between Toshiba Machine Co., Ltd. and QED Technologies (QED) to manufacture representative examples of glassy carbon. Details of the grinding, pre-polishing and final polishing process will be provided along with the resultant metrology results after key steps. Molding experiments based on these developments will also be presented.

The Near Infrared Camera (NIRCam) for NASA's James Webb Space Telescope (JWST) is one of the four science instruments to be installed into the Integrated Science Instrument Module (ISIM) on JWST. I-220H beryllium was chosen as the optical bench material for NIRCam based on its high specific stiffness, relatively high thermal conductivity, low CTE at cryogenic temperatures, and overall thermal stability at cryogenic temperatures. Beryllium has cryogenic heritage, but development of a structural bonded joint that could survive cryogenic temperatures was required. This paper will describe the trade studies performed in which bonded, I-220H beryllium was selected.

To increase margins over requirements and improve structural performance, it was desired to reduce the weight of the optical bench in the Laser Transmitter Unit (LTU) of the Rapid Airborne MIne Clearance System (RAMICS) program. First-mode response for both the optical bench and bench/housing combination is of critical importance to the stability and long-term field performance of this system. Several candidate materials were reviewed and analyzed against the previous design in 7075 aluminum. AlBeMet AM162 was chosen for its high Young's modulus, lower density, and a number of other factors as discussed. Finite element modeling is presented which was used in the downselect process. An optical bench weight reduction of 50% was achieved due to material and design changes, as well as an improvement in the first-mode frequency of the laser transmitter based on analysis. Vibration testing results and a comparison of fabrication costs are also presented.

Adaptive materials integrate actuating and sensing materials into a structural material. The development of polymer composites with embedded shape memory alloys can open new perspectives with respect to the development of engineering structures with adaptive shape, stiffness, damping and other properties. The development of these advanced composites with embedded shape memory alloy wires is still in an embryonic stage. Given ellipsometry measurements of optical properties copper based alloy which require further research before these adaptive composites can be used in industrial applications.

Optical properties of flawless bulk copper sample has been studied based on spectroellipsometry measurements in wide spectral interval (hv=0.18-4.87 eV) at various angles of light incidence. The main characteristics of electronic subsystem of this metal and values of main energy intervals in a band structure were determined for both oxidized and unoxidized copper sample.

Keywords/Phrases

Keywords

in

Remove

in

Remove

in

Remove

+ Add another field

Search In:

Proceedings

Volume

Journals +

Volume

Issue

Page

Journal of Applied Remote SensingJournal of Astronomical Telescopes Instruments and SystemsJournal of Biomedical OpticsJournal of Electronic ImagingJournal of Medical ImagingJournal of Micro/Nanolithography, MEMS, and MOEMSJournal of NanophotonicsJournal of Photonics for EnergyNeurophotonicsOptical EngineeringSPIE Reviews